1. Field of the Invention
The invention relates to a glenoid prosthesis comprising a plastic bearing shell with a plurality of integral anchoring pins, which are arranged parallel to one another at its reverse side.
2. Description of the Prior Art
The operation technique and the material used are described in detail in a brochure: xe2x80x9cSchulter-Endoprothese zementiertxe2x80x9d, 1998 edition of Sulzer Orthopedics Ltd., CH-6340 Baar. A glenoid prosthesis with four pins on the reverse side is shown which is cemented in after the milling and boring of the natural glenoid.
The object of the present invention is to be able to insert a glenoid prosthesis of this kind in a wider spectrum of applications. This object is achieved by the present invention in that at least one anchoring pin has a thread as a coupling element, and in that a metal sleeve with protruding longitudinal ribs is solidly screwable to that one anchoring pin for a mechanical hammering in of the sleeve into an undersized bone hole.
This design of the glenoid prosthesis brings about a plurality of advantages. It permits the prosthesis to be selectively cemented in or to be anchored directly in the bore without cement after the securing of sleeves with anchoring structures. In the cementing in, the coupling structure at the pins has the advantage that projections and recessions are present for the cement jacket and that no overloading takes place since cement can flow back into depressions. In the hammering in without cement a sleeve material which is suitable for the growing in and an anchoring structure on the sleeve which is suitable for the hammering in can be used.
Other embodiments of the present invention provide advantages. It is advantageous if the pins have an outer thread as a coupling element and the sleeves have an inner thread as a fitting coupling element. Bayonet locks are likewise possible. In addition it is advantageous if all pins are provided with equally large thread diameters independently of the size of the bearing shells since only one additional part, the sleeve, need be taken up into the bearing holder. Furthermore, a bearing shell with pins of a material which has good sliding properties can be chosen without it being necessary for the anchoring to suffer thereby. A construction of plastic in the form of a bearing shell of polyethylene combined with metallic sleeves of a titanium ally brings about a good anchoring. This can be improved if knife-like ribs in the longitudinal direction are distributed over the periphery of the sleeve. On the one hand, the support surface is enlarged and, on the other hand, the bone material is encouraged to grow in. Furthermore, the growing in of bone matter can be improved if the reverse side of the bearing shell is provided with a metal mesh which for example consists of a titanium alloy.
A further advantage results if the orthopedist can still make the decision, after the boring of the bores for the anchoring pins, in accordance with the state of the bone found, whether he wishes to cement or to hammer in without cement. This is achieved in that the sleeve has such a low wall thickness that the bore diameter, which corresponds to a desired thickness of the cement jacket during the cementing, at the same time also forms the desired bore diameter for the direct hammering in of sleeves with an anchoring structure. That is, the anchoring structure is designed in such a manner relative to the pin that the bore diameter is less than the outer diameter over the anchoring structure and lies in the vicinity of the outer diameter of the actual sleeve.